This application claims priority to Korean Patent Application No. 2004-77157, filed Sep. 24, 2004 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to differential amplifiers, and more particularly, to a differential amplifier having class AB control with cascode loading for increased gain and reduced offset.
2. Description of the Related Art
A push-pull amplifier circuit including CMOS (complementary metal oxide semiconductor) transistors is an often used circuit. An audio amplifier circuit is typically an analog amplifier circuit or a digital amplifier circuit.
An analog amplifier circuit is classified into one of a class A amplifier circuit, a class B amplifier circuit, or a class AB amplifier circuit, and a digital amplifier circuit is classified as a class D amplifier circuit. Since linearity is considered more important than high efficiency in an audio amplifier circuit, an analog amplifier which is a linear amplifier is generally used for the audio amplifier.
The class A, B, and AB amplifier circuits have recently been used as analog amplifier circuits because of their higher linearity. However, such amplifiers consume relatively high power for generating high output. That is, analog amplifiers have the advantage of high linearity, but the disadvantage of low efficiency.
Specifically, in a class A analog amplifier, much more power is dissipated than the maximum output of the amplifier, and therefore the efficiency of the amplifier is typically not more than 25%. The class B push-pull amplifier overcomes the efficiency problem of the class A amplifier with two transistors coupled to each other in an emitter follower configuration. However, the class B push-pull amplifier has crossover distortion for low signal levels.
Further, although the transistors of the class B push-pull amplifier are easily turned on and off for small currents, such transistors cannot be rapidly turned on and off for large currents. More specifically, since no bias current flows in the class B amplifier when in an idle (i.e., static) state, it is difficult to rapidly turn the transistors on/off for large currents resulting in increased total harmonic distortion (THD).
In the class AB amplifier, a small amount of current flows through the amplifier in the idle state. Such current is much smaller than in the class A amplifier but larger than in the class B amplifier. That is, with higher amount of current in the idle state, the class AB amplifier becomes more similar to the class A amplifier, and with lower amount of current in the idle state, the class AB amplifier becomes more similar to the class B amplifier.
U.S. Pat. No. 5,311,145 to Huijsing et al. describes a floating class AB control terminal in a differential amplifier. A class AB control terminal controls a cross over point of the differential amplifier with a quiescent current in the idle state in order to prevent distortion of an output signal. Further, high gain is maintained with positive feedback.
Control of currents in a differential amplifier is desired with high amplifier gain in both static and active operating states.